1. Field of the Invention
The present invention relates to network communications, and more specifically, automatic protection switching in network communications.
2. Description of the Related Art
A collection of interconnected networks is an “internet.” An internet is formed when separate networks are connected together. The specific worldwide computer internet, the “Internet” refers to a global computer network that consists of internetworked communications systems. Computer networks such as the Internet use a protocol hierarchy organized as a series of layers. The combination of layers and protocols for a given computer network is referred to as a network architecture. Each layer may have a protocol governing the method of communication for the layer. A protocol refers to rules that govern the format of frames and packets that are exchanged via a computer network to a peer entity. One such protocol, “IP” (Internet Protocol) defines the packet format for a network layer.
Another layer of a computer network is the physical layer. The physical layer relates to the transmitting over a communication channel and concerns the physical transmission medium. One physical medium used for computer networks today is optical fiber. The optical fiber standard used for most long-distance telephone connections is Synchronous Optical NETwork (SONET).
A similar standard to SONET is the Synchronous Digital Hierarchy (SDH) which is the optical fiber standard predominantly used in Europe. There are only minor differences between the two standards. Accordingly, hereinafter any reference to the term SONET refers to both SDH and SONET networks, unless otherwise noted.
SONET provides a standard for multiplexing multiple digital channels together. SONET is a time division multiplexing system that devotes the entire bandwidth of the fiber to one channel that contains time slots for various subchannels. Accordingly, it is a “synchronous” system. A SONET line transmits digital information in bits at precise intervals. SONET systems are made up of switches, multiplexers and repeaters. In SONET terms, the transmission between two endpoints can be conceptually broken into a hierarchy of sub-parts, comprising of a “section,” the fiber connection directly connecting one device to another device; a “line,” the fiber connection between any two network elements that terminate lines, i.e., line terminating equipment (LTEs) including multiplexers; and a “path” the fiber connection between a source and a destination that terminates in path terminating equipment (PTEs). A line consists of several sections, and a path consists of several lines. Topologically, a SONET system can be either a mesh or a dual ring. Note that the term “line” used in a SONET network is equivalent to the term “section” in a SDH network.
Data transmitted according to the SONET standard is in frames, which, according to one standard, are organized as blocks of data with a block of 810 bytes transmitted every 125 μsec. The number of bytes transmitted per frame varies with the transmission rate of the connection, but in all cases frames are transmitted at a rate of 8000 frames per second which matches the sampling rate of pulse code modulation channels that are used in digital telephony systems.
The SONET system specification includes provisions for automatic protection and reconfiguration in case of failure, called APS (Automatic Protection Switching) in the SONET specification, and MSP (Multiplex Section Protection) in the SDH specification. An APS/MSP configuration could include one ‘protect’ interface circuit and one ‘working’ interface circuit. One such APS configuration is known as “1+1 linear APS”. In a 1+1 linear APS configuration, any data transmitted by a network element is transmitted to both the working circuit and the protect circuit connected to the network element.
When routers are configured as network elements in a SONET system configured for APS, the interface circuits connecting the routers to the working and protect circuits may be configured to be located in separate routers or the same router. Protection occurs at the SONET line level (in SDH terminology, protection occurs at the SDH section level). Protection control bytes transmitted between LTEs, the end points of a SONET line, communicate APS protection information. This protection information includes whether the protect or working circuit is currently active. Normally, when all equipment is functioning correctly, traffic is carried by the working circuit. In APS configurations involving routers, the working interface is active and the protect interface is inactive. If the working circuit fails, an “APS switch” occurs, causing deactivation of the working circuit and activation of the protect circuit. In APS configurations involving routers, the working interface is deactivated and the protect interface is activated.
The transmissions between LTE are synchronized by the protection information transmitted in the protection control bytes. Some SONET/SDH multiplexers do not comply with the SONET/SDH standards, and do not appropriately transmit protection control bytes to the other end of the line segment. Such cases preclude implementation of protection logic by the local router.
What is needed is a system capable of implementing protection logic on SONET/SDH lines connecting routers to multiplexers that do not transmit protection control bytes on the SONET/SDH line.